The cucurbit[n]uril (CB[n]) family has been extensively studied in recent years on account of their scope within the fields of molecular recognition and aqueous self-assembly.[1] CB[7], a member of the CB[n] family, has a substantially higher water solubility than CB[6] and CB[8] and a larger binding cavity than its water soluble brother, CB[5].[2]
Recent reports have shown that CB[7] is an excellent host for binding and stabilising a wide range of small molecules, such as fluorescent dyes,[3] controlling aggregate formation of guest molecules,[4] and for use in light-harvesting systems.[5] However, a major drawback which has limited the study and application of CB[7] is its difficult isolation and purification from the synthesised mixture of CB[n] homologues.
Traditional isolation methods of CB[7] are based on the different solubilities of CB[n] in various solvents, such as acetone/water and methanol/water mixtures, limiting the overall yield of CB[7] to just under 3%.[6, 7] Nau et al. [8] has reported a more direct approach to preparing CB[7] by controlling the direct reaction of formaldehyde, glycoluril and sulfuric acid, followed by a similar isolation methodology based on solubility of CB[7] in an acetone/water mixture. This method gives a substantially better yield with higher purity, however, the separation and purification is still based primarily on solubility differences in a variety of mixed solvents. For the standard separation routes, it is difficult to obtain CB[7] in high purity as both CB[5] and CB[7] have low solubility in the mixed solvent systems (either acetone/H2O or MeOH/H2O), requiring multiple solvent-based separation cycles.
Another isolation method which has been reported by Isaacs et al. uses column chromatography with a harsh acidic eluent HCO2H (88%):HCl (0.2 M) (1:1 v/v).[9] This can be a highly toxic process which results in acidic products and may limit the viability of scale-up.
The use of alkyl imidazolium salts to form complexes with cucurbituril compounds has been described [18], although this disclosure was made after the priority date of the present application.
Herein is reported a new cucurbituril isolation method which is convenient for obtaining pure cucurbituril material, is inherently more environmentally friendly and suitable for large scale purification by exploiting the reversible association and dissociation of a host-guest complex in a controlled manner.